Power supply circuits are known for multicolour LED systems wherein a single LED current generator is connected to a plurality of subsets of LEDs. The LEDs may, for example, have different colours. In order to control the brightness of the different colour LEDs, and therefore the overall appearance of the LED display, the subsets of LEDs are driven with different levels of power. This may be conveniently done by supplying each subset of LEDs with a pulse modulated signal. The pulse modulated signals are typically provided by a current generator. The current generator must be able to supply sufficient power to multiple subsets of LEDs when the pulse modulated signals have overlapping on-times; conversely, there may be periods during which the pulse modulated signals have overlapping off-times: at these overlapping off-times, the current generator is not required to provide power.
Pulse modulation of the individual subsets of LEDs is conveniently effected through the provision of bypass switches. Thus, an individual subset of LEDs is powered when the associated bypass switch is open; however, when the associated bypass switch is closed the signal bypasses the subset of LEDs, which are in consequence not operational. Such bypass switches are disclosed, for instance, in PCT patent application publication WO2007/069200.
In order to improve the overall efficiency of the power supply, it is known to switch off the current generator during periods when all the bypass switches are closed and thus no current is required by the load. PCT patent application publication number WO2006/080365 discloses a power supply providing non-overlapping pulse modulated signals to multiple subsets of LEDs, in which the current generator is disabled during a non-light-emission period when none of the light emitting diodes are driven by the drive controller. Such a circuit has the disadvantage that, when the current is required, the generator has a non-zero rise time for the current. In other words, during the period when the current is first required by one of the subsets of LEDs, the current generator can only provide a fraction of the required current. This results in a noticeable dimming, or flicker, of the LED light source display.